1. Field of the Invention
The present invention relates generally to a projection exposure apparatus and method employed in a photolithography process of manufacturing a semiconductor element and a liquid crystal display element, etc. and, more particularly, to a projection exposure apparatus and method capable of focusing on a photosensitive substrate.
2. Related Background Art
In a conventional projection exposure apparatus, when imaging a pattern of a mask (reticle) on a photosensitive substrate (wafer) through a projection optical system, it is indispensible to align a wafer surface with a pattern imaging plane, i.e., to perform focusing. In recent years, a focal depth of the projection optical system has become smaller. On the other hand, even an apparatus using t-line of a wavelength of 365 nm for an exposure illumination can obtain only a focal depth on the order of .+-.0.7 .mu.m. This is the real situation. Accordingly, a focus detection system incorporated in this type of projection exposure apparatus is required to highly accurately detect a deviation quantity between the imaging plane of the projection optical system and the wafer surface in an optical-axis direction of the projection optical system.
As described above, a technique disclosed in, e.g., U.S. Pat. No. 4,558,949, is known as a method of performing the focusing by detecting the deviation quantity between the imaging plane of the projection optical system and the wafer surface in the optical-axis direction of the projection optical system with the high accuracy. According to this method, the wafer is irradiated with a laser beam (non-photosensitive to a resist on the wafer) in the form of a pattern image. The light beam reflected therefrom is photoelectrically detected by a synchronous detection method. Then, there is highly accurately detected the deviation quantity between the imaging plane of the projection optical system and the wafer surface in the optical-axis direction of the projection optical system.
Herein, it is assumed that the optical-axis direction of the projection optical system is termed a Z-direction, and a coordinate system for prescribing a moving position of a stage within a plane perpendicular to the Z-direction is termed an XY-coordinate system. When detecting a height position of the wafer surface, an image forming position (an XY-coordinate position) of a pattern image (hereinafter referred to as a slit image) projected on the wafer by the focus detection system is previously adjusted, wherein a position of the optical axis of the projection optical system is a set position within the imaging plane of the projection optical system. Hence, the focus detection system detects the focal point by projecting the slit image on a point-of-center of each shot area which has already been exposed on the wafer.
In the conventional projection exposure apparatus, however, the slit image is adjusted to fall within a predetermined allowable range in which the above set position on the imaging plane of the projection optical system is centered. Accordingly, there is a scatter in terms of the coordinate position in which the slit image is actually formed on the imaging plane of the projection optical system according to each projection exposure apparatus.
In general, the surface of each shot area exposed on the wafer is formed with a stepped portion (a ruggedness) through an exposure process. A scribe line may be exemplified as this ruggedness. When forming two circuit patterns within one shot area, this implies a formation of a groove existing between these two circuit pattern areas. A groove width is approximately 2 mm, and a groove step is on the order of 1-5 .mu.m.
In the case of effecting the focus detection by making the slit image fall on the above shot area, the above-mentioned conventional apparatus can use the following techniques; the slit image is made to fall on the point-of-center of the shot area, i.e., on the scribe line; and the slit image is made to fall on a position off the point-of-center of the shot area, i.e., not on the scribe line but on the circuit pattern area. A height difference as large as 5 .mu.m at the maximum is produced in the focus detection, depending on the apparatus. In such an exposure apparatus, it is difficult to dispose the wafer surface within a focal depth as small as .+-.0.7 .mu.m. Further, a micro stepped portion exists even in the shot area with no scribe line, and this also exerts a large influence on the focus detection.
Further, in the conventional projection exposure apparatus, there is a scatter in terms of the imaging status (in-focus status) of the slit image on the wafer according to each projection exposure apparatus, and the focal point of the slit image is not always the most appropriate on the wafer. There is a problem that an out-of-focus state of this slit image causes an error in focus detection.